It is well known that olefins can be polymerized with a catalyst composition comprising a support such as silica with a chromium oxide associated with and carried by the support. These olefins have from 2-8 carbon atoms and are 1-olefins. Also these olefins may be copolymerized with 1-olefins of 2-20 carbon atoms. The catalyst of this invention which uses a substantially pure silica free of alkali metal or alkaline earth metal ions is produced by the hydrolysis of 1 or more orthosilicate esters in a water solution containing mixed solvents and the suspended silica so produced is separated from the liquid and particularly from the water and then associated with a chromium compound that is either a chromium oxide or a compound convertible to an oxide during the processing of the catalyst.
This improved catalyst is useful in the particle form process of polymerizing and copolymerizing olefins and the resulting particle form polyethylene, for example, is of high melt index and improved molecular weight distribution that is suitable for injection molding applications. In addition, the new catalysts of this invention have melt index-temperature dependence different from those of existing catalysts of which we are aware. Furthermore, the catalysts of this invention can be modified by fluoride treatment as taught in U.S. Pat. No. 3,130,188 and when so modified have melt index-temperature dependence different from that of the fluorided catalysts presently available.
In addition, the catalysts of this invention can be used to produce extremely high melt index particle form polyethylene by the method described in the Hoff et al copending application Ser. No. 633,507, filed Nov. 19, 1975, now U.S. Pat. No. 4,041,224 and assigned to the assignee hereof.